This invention relates to differential phase shift keyed modulators, and more particularly, to digital differential phase shift keyed modulators which use differential phase shift modulation to encode a binary data input such that the encoded signal can be transmitted over telephone lines.
Prior art differential phase shift keyed (DPSK) modulators, such as Bell System Data Sets 201A and 201B, operate at a fixed data input rate and are compatible only with the U.S. system of phase shift modulation. These prior art devices require amplitude modulators, low pass filters and summing amplifiers which utilize linear circuitry. Due to the presence of these linear circuit elements the prior art modulators require adjustment and calibration. Additionally, since these prior art devices are not totally digital they cannot be constructed on a single large scale integrated circuit chip.
The present inventive device is constructed using only digital circuitry enabling the entire apparatus to be constructed on a single large scale integrated circuit chip. Since the circuit is entirely digital, adjustments and calibration are not required. The system as constructed is fully compatible with either the U.S. phase shift encoding convention or the European (CCITT) phase shift encoding convention. The present system additionally is capable of operating at either of two input data rates which can be selected at will. Where prior art devices required linear amplitude modulators, low pass filters and summing amplifiers, the present invention performs all three of these tasks with a single read only memory (ROM) and associated circuitry.